Potentiation of retinoic acid-induced U-937 differentiation into respiratory burst-competent cells by nitric oxide donors

All- trans retinoic acid (tretinoin) is a known inducer of differentiation of the human monoblastic cell line, U-937. We now report that the ability of retinoic acid (RA) to induce differentiation of U-937 cells into cells possessing respiratory burst activity is enhanced by the known nitric oxide-donating drugs glyceryl trinitrate, molsidomine and CAS 936, and by tetranitromethane in combination with cysteine. RA alone was a strong inducer of U-937 differentiation as indicated by the following responses to 12- O-tetradecanoylphorbol-13-acetate (TPA) stimulation: (1) increase in the percentage of cells staining with nitroblue tetrazolium (NBT); (2) increase in the total amount of formazan (the product of NBT reduction by O 2 −) as determined spectrophotometrically; (3) increase in hexose monophosphate shunt (HMPS) activity as assessed by [ 14C]CO 2 released from d-[1- 14C]glucose. RA was also able to increase mRNA levels for two respiratory burst-related genes and for glucose-6-phosphate dehydrogenase (G6PD), an HMPS enzyme. Other indications of differentiation were reduced cell proliferation, increased adherence and altered nuclear morphology. The observed increase in formazan production and HMPS activity and the reduction of cell proliferation due to RA were augmented by co-treatment with either glyceryl trinitrate, molsidomine, CAS 936 or tetranitromethane plus cysteine. Glyceryl trinitrate alone increased HMPS activity and G6PD mRNA levels and also reduced cell proliferation. Glyceryl trinitrate, molsidomine and CAS 936 are presumed to release nitric oxide and increase intracellular cGMP levels by stimulation of soluble guanylate cyclase. The mechanism of action of tetranitromethane is less certain, although it may also generate reactive nitrogen intermediates. These data suggest that a NO·/ cGMP pathway may augment a retinoic acid-mediated pathway to enhance maturation of U-937 cells with respect to the respiratory burst. Glyceryl trinitrate may act additionally by another pathway.